Method for removal of ammonia from a gas mixture

ABSTRACT

The present invention relates to a method of catalytic production of ammonia, where a gas mixture consisting mainly of unconverted synthesis gas, some ammonia, inert gases and possibly also water is recirculated to the synthesis reactor being first freed from the main part of the ammonia and possibly also water by absorption. The gas mixture is brought in contact with a hygroscopic abosrption agent having two or more OH-groups for absorption of ammonia and possibly also water. The absorption is carried out at a pressure being substantially the same as the ammonia synthesis pressure, and ammonia being desorbed from the absorption agent at a lower pressure and higher temperature than at the absorption. At least part of the possibly present water is removed from the absorption agent before it is supplied to the absorption column. Preferably it si applied ethylene glycol, diethylene glycol or triethylene glycol alone or in mixture as absorption agent. Possible water can be removed form the absorption agent by inert stripping gas before it is returned to the absorption column.

BACKGROUND OF THE INVENTION

The present invention relates to a method for removal of ammonia from agas mixture formed by catalytic production of ammonia. The gas mixtureconsists mainly of unconverted synthesis gas, some ammonia, inert gasesand possibly also water. Ammonia and possibly also water are removedfrom the gas mixture by absorption with an organic absorption agenthaving two or more OH-groups, preferably a glycol.

Ammonia is produced by catalytic reaction between nitrogen and hydrogen.A commonly used synthesis pressure is about 200 bar. The conversion isnot complete and unconverted synthesis gas has to be recirculated to thereactor. The ammonia formed must accordingly be separated from the gasmixture. Some ammonia can be recirculated. The problem is to remove asmuch of the ammonia as possible in an economical way, and it isespecially important to have effective ammonia removal when thesynthesis is carried out at relatively low pressure.

Another problem is the water which gets into the gas mixture fromearlier stages in the process, especially the methanation stage. Thewater in the synthesis gas will deactivate the catalyst. Accordingly, itis desirable to remove as much as possible the water which might bepresent, before the gas reaches the synthesis reactor.

Ammonia can be removed from the gas mixture in several ways. The mostused one, especially in high pressure units, is to remove the ammoniafrom the synthesis gas by condensation with cooling water. However, onlypart of the ammonia will be removed in each recirculating step. If lowersynthesis pressure is used, a cooling unit requiring a high input ofenergy to its compressor is used, and the costs of apparatus utilized,for instance heat exchangers, will be high.

Another method which has been proposed is to wash out the ammonia withwater, for instance as described in GB 2.067.175 A. The ammonia can beseparated from the water by distillation. The main disadvantage of thismethod is that the gas mixture is moistened by the water. The result ofthis is that the gas mixture has to be dried subsequent to the ammoniaremoval to avoid deactivation of the catalyst. A molecular sieve is usedfor removing water from the gas mixture.

Another disadvantage of this type of process is that the heat ofabsorption and heat of desorption for ammonia in water is high. Theresult of this is that a large amount of energy has to be supplied forseparation of the ammonia from the water.

Further it is known from DE 19 24 892 to remove ammonia from partlyconverted synthesis gas by absorption in a solvent, for instanceethylene glycol. The solvent can be regenerated by heating with steam.Desorption is carried out in a column having a lower pressure.

The disadvantage of this method is that desorption is carried out undersuch low pressure that the cooling water can not be used in thesubsequent condensation of ammonia. Ammonia must therefore be compressedor re-absorbed in order to get liquid ammonia, and this is expensive.

Another problem with this method is that the desorption temperature whenusing glycol has an upper limit of 170° C. because of the danger ofdecomposition. If high is applied pressure during the desorption suchthat it is possible to use cooling water for condensation of ammonia,the solvent will not be completely regenerated and accordingly part ofthe ammonia will be recirculated back to the synthesis.

SUMMARY OF THE INVENTION

The object of the present invention was to arrive at an economical wayof removing as much as possible of the ammonia formed from unconvertedsynthesis gas in a gas mixture which shall be returned to the ammoniasynthesis. It was especially desired to remove ammonia at conditionswhich make it possible to effectively utilize cooling water forcondensation of ammonia.

The inventors aimed at purifying the gas mixture which should bereturned to the ammonia synthesis to such a degree that the synthesiscould be carried out at a relatively low pressure and withoutsubstantial deactivation of the catalyst. In order to obtain this it wasexpected that the concentration of ammonia in the gas mixture should bebelow 0.5 volume %.

It was found that if this goal should be attained, it would be quiteexpensive and energy consuming to separate all ammonia by condensation.It is known that the solubility of ammonia in water is very high, butalso that it is uneconomical to wash out/absorb ammonia by water forinstance because the gas mixture had to be dried for each circulationwhen extra water is brought into the gas mixture during the ammoniaabsorption stage. Removal of ammonia by some form of absorption ishowever considered as a possible method, and it will then be a questionof finding an absorption agent which will result in a totally economicalprocess. It was therefore attempted to define a suitable absorptionagent and one arrived at the following requirements was determined forsuch agent:

high solubility for ammonia;

result in a simple and low energy consuming separation of the absorptionagent and ammonia;

low volatility at the actual pressure and temperature;

it should not deactivate the catalyst, nor should it be poisonous withrespect to health.

it has to be stable and not decompose;

corrosion problems should be as small as possible; and

its price should be as low as possible.

It was further important with regard to a most economical process thatthe following considerations were made:

the temperature in the absorption tower should be as low as it ispossible to obtain by using cooling water;

the pressure in the desorption tower should be as low as possible, butstill sufficiently high for utilizing cooling water in the condenser;

the temperature at the bottom of the desorption column should be as highas present low energy heat from other parts of the ammonia units allows,for instance 100° -150° C.; and

when ammonia is to be removed from the absorption agent, the highestpossible temperature shall be applied in the desorption column withoutdamaging (decomposing) the absorption agent in order to remove as muchof the ammonia as possible in the desorption column.

Possible absorption agents and process conditions were then investigatedwithin the above criteria for the absorption agent and process.Separation of ammonia from other gas mixtures was also investigated inorder to find out whether agents used or process conditions could beapplied for solving the actual problems.

Organic absorption agents having two or more OH-groups, for instanceglycol, seemed in the beginning to meet at least some of the aboverequirements. Despite the disadvantages which the process according tothe above mentioned DE 1924892 has, the inventors still found that theywould try to use this type of absorption agent, but then alter theprocess to utilize low value process heat and cooling water present.

Further investigations of suitable absorption agents within the abovementioned type showed that, first of all, high boiling glycol,especially diethylene glycol, could be applicable.

The inventors then studied how to carry out the absorption anddesorption with regard to utilization of cooling water. It was thenfound that by carrying out the absorption at a pressure which issubstantially the same as the pressure of the ammonia synthesis and thendesorbing ammonia from the desorption agent in at least two stages, onewould be able to utilize cooling water in an economical way. It was thenalso possible to remove ammonia efficiently so that the gas mixturereturned to the synthesis contained less than 0.5 volume % ammonia. Itwas further found that it was most practical to desorb the main part ofthe ammonia in the first stage at 7-20 bar, condense with cooling waterat a temperature of 5°-35° C., then in a subsequent stage desorb ammoniaat a pressure of 1-3 bar. Energy could be saved by using furtherdesorption stages at 3-15 bar.

The water which can be present in the gas mixture originates mainly fromthe methanation step. This water can be removed in several ways knownper se and at different places in the process. Thus it can for instancebe removed ahead of the absorption column by means of glycol or from theabsorption agent ahead of the desorption column. But it can also beremoved from the purified extractant before it is returned to theabsorption column.

The invention will now be further explained in connection with thedescription of the figures and the examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow-sheet for removal of ammonia from a partly convertedsynthesis gas from an ammonia reactor.

FIG. 2 shows a flow-sheet for removal of ammonia from a partly convertedsynthesis gas and where the ammonia is removed as a product atatmospheric pressure.

FIG. 3 shows a flow-sheet for removal of ammonia from partly convertedsynthesis gas and where the ammonia is removed as a liquid product atcooling water temperature.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 is shown the gas mixture 1 which shall be freed of its ammoniacontent as much as possible and the purified synthesis gas mixture 17which is transferred back to the ammonia reactor (not shown). Completelyregenerated absorption agent 21 is supplied to the absorption column 2via pump 22, and partly regenerated absorption agent 13 is suppliedthrough pipe 3 via pump 14. The temperature in the column 2 is regulatedby means of cooling water 4. The upper part 15 of the column is cooledby cooling water 16. The absorption agent containing absorbed ammonia istransferred out of the bottom of the column 2 through pipe 5 through theheat exchanger 6 and the pressure reduction valve 7 to the desorptioncolumn 8. Ammonia 11 is removed from the top of the column 8, while someof the ammonia is condensed in cooler 10 where some inert gas can alsobe removed. Ammonia is transferred in reflux via pipe 12 back to the topof the column 2. The liquid from the column 8 is heated in heatexchanger 9 and the gas is returned to column 8. The liquid from theheat exchanger 9 is partly regenerated absorption agent 13. The mainpart of the absorption agent 13 is transferred in pipe 3 through heatexchanger 6 back to the absorption column 2.

Some of the partly regenerated absorption agent 13 is transferredthrough pipe 18 and reduction valve 19 to a separating tank 20. Theabsorption agent is regenerated there. The regenerated absorption agent21 is returned to the absorption column 2. The desorbed ammonia 23 fromthe separation tank 20 is first cooled by cooling water 25 in the cooler24. Condensed absorption agent 26 is transferred to pipe containingregenerated absorption agent 21. Desorbed ammonia 23 is compressed incompressor 27 and transferred to the desorption column 8.

FIG. 2 shows an embodiment of the invention where ammonia finally isremoved as liquid at atmospheric pressure. Ammonia from the top of thedesorption column 8 is cooled and transferred by pipe 11 via pressurerelief valve 28 to a separation tank 29. The liquid ammonia product 30is removed at the bottom of the separation tank 29. Gaseous ammonia fromthe separation tank 29 is transferred in pipe 31 through heat exchanger32 to the compressor 27. Non-condensed gas 33 from the cooler 10 iscooled in cooler 32 and transferred to separation tank 34. Inert gasesare removed through pipe 35 and condensed liquid is transferred in pipe36 to a reduction valve 37 to the separation tank 29. The remaining partof the process is as shown in FIG. 1.

FIG. 3 shows an alternative way of treating desorbed ammonia fromseparation tank 20. Ammonia is transferred to the absorption tank 26 andcooled by cooling water 27. The absorption agent is supplied throughpipe 24 and is cooled in cooler 25. The liquid 28 from the absorptiontank 26 is pumped back to the desorption tower 8 via pump 29 and heatexchanger 25.

EXAMPLE 1

This example shows the method according to the invention carried out ina unit as described in FIG. 1.

Gas mixture 1 at 25° C., 50 bar and containing 10 volume % NH₃ iscontacted by the ethylene glycol containing some ammonia in column 2.Regenerated absorption agent (0.8 weight % NH₃) 21 is supplied to thetop of the column, and partly regenerated absorption agent 13 (6.8weight % NH₃) is supplied at the center of column 2. Cooling water 4 and16 ensure that the temperature in the absorption tower is kept at about25° C. The ammonia content of the liquid at the bottom of the tower 5became about 16 weight %.

The stream 5 from column 2 is warmed in heat exchanger 6, and thepressure is relieved in valve 7 to 11 bar and then to stream 5 istransferred to desorption column 8. Liquid at the bottom of the columnis heated in the boiler 9 at about 130° C. Ammonia at the top of thedesorption column 8 is condensed in condenser 10 at about 25° C. Inertgas can also be removed in the condenser. Some of the ammonia isreturned to the absorption column as reflux and the rest is productammonia at 25° C.

The liquid stream 13 from the boiler 9 contains 6.8 weight 5 ammonia.The stream 3 is cooled in the heat exchanger 6 and pumped by pump 14back to the center of the absorption column 2.

The liquid stream 18 from the boiler is pressure relieved by valve 19 to1 bar and transferred to the desorption tank 20. Ammonia 23 from the topof the desorption tank 20 is cooled in cooler 24 at about 25° C.,compressed to 11 bar and transferred to desorption column 8. Condensate26 from cooler 24 is transferred together with the liquid stream fromthe desorption tank 20. Liquid from the desorption tank 20 containingabout 0.8 weight % ammonia is pumped by the pump 22 to the top of theabsorption colum 2.

The ammonia concentration at the top of the absorption column becomes0.5 volume %. This means that about 95% of the ammonia in the stream 1is absorbed and is removed as product 11.

EXAMPLE 2

This example shows the method according to the invention carried out ina unit as described in FIG. 2.

The liquid ammonia stream 11 from Example 1 is at 25° C. By this processthe pressure of the stream 11 is reduced to 1 bar in valve 28 and theliquid is transferred to separation tank 29. The temperature in the tank29 will then be -33° C. The ammonia gas from the separation tank 29 isheated in heat exchanger 32 and transferred together with the stream 23to the compressor 27. Non-condensed gas from cooler 10 is cooled in theheat exchanger 32 and transferred to separation tank 34. The gas stream35 from the separation tank 34 contains minor amounts of nitrogen,methane and argon. Condensed liquid from separation tank 34 is expandedin the valve 37 and transferred to separation tank 29. The ammoniaproduct 30 will be a liquid at -33° C. The ammonia concentration in thegas mixture returned to the synthesis was 0.5 volume %. This means thatabout 95% of the ammonia in the gas mixture from the synthesis wasabsorbed and removed as a product 30.

EXAMPLE 3

This Example shows the method according to the invention carried out ina unit as described in FIG. 3. The difference between this method andthat of Example 1 is found in the way the ammonia gas is transferredback to the process.

The method is the same as in Example 1 except that the boiler 9 isoperated at about 105° C. such that partly regenerated diethylene glycolcontains about 5.3 weight % NH₃. From the separation tank 20, theammonia gas 23 is transferred to the absorption tank 26. Partlyregenerated diethylene glycol 24 is cooled in the heat exchanger 25 andtransferred to the absorption tank 26. The absorption tank 26 is cooledby cooling water 27 such the temperature is kept at about 25° C. Liquid28 from the absorption tank 26 containing about 8.5 weight % NH₃ ispumped up to 11 bar by pump 29, heated in the heat exchanger 25 andtransferred to the absorption column 8.

This way of carrying out the method results in removal of 98% of theammonia in the gas mixture from the synthesis by absorption and theammonia is removed as product 11. The ammonia concentration in the gasmixture returned to the synthesis was 0.2 volume %.

By the present invention, one can in a simple and economical way removeat least 95% of the ammonia in the synthesis gas mixture before it isreturned to the ammonia reactor, this means that the gas mixture wouldcontain less than 0.5 volume % ammonia. Further, water present in thegas mixture can be reduced to about 1 ppm. The consequence of thesemeasures is that such a pure synthesis gas is supplied to the reactorthat the ammonia synthesis can be carried out in an economical way atsubstantially lower pressure than previously has been possible by knowntechniques.

A by-effect of the method according to the invention is that processingof the purged gas (inert gas removal) can be carried out in a simplerway than usual because the ammonia content is low. The method willfurther be suitable if it is desired to bring the water content below 1ppm in the gas mixture before it is returned to the ammonia synthesis.

We claim:
 1. In a method for removal of ammonia from a gas mixtureformed in an ammonia synthesis process by catalytic production ofammonia, wherein the gas mixture comprises unconverted synthesis gas,ammonia, and inert gases, and wherein the gas mixture is freed fromammonia by absorption with an organic absorption agent having two ormore OH-groups, the ammonia is subsequently desorbed from the absorptionagent, and the resultant gas mixture is then returned to the ammoniasynthesis process, the improvement comprising:carrying out theabsorption at a pressure which is substantially the same as the pressureof the ammonia synthesis process; and carrying out the desorption of theammonia from the absorption agent in at least two stages, including afirst desorption stage in which a major portion of the ammonia isdesorbed at a pressure of 7-20 bare and condensed by cooling water at atemperature of 5°-35° C., and a second desorption stage in which furtheramounts of ammonia are desorbed from the absorption agent at a pressureof 1-3 bar.
 2. A method according to claim 1, whereinthe desorption iscarried out during heating by process heat at a temperature of 100°-150C.
 3. A method according to claim 1, whereinammonia desorbed in thesecond desorption stage is compressed and returned to the firstdesorption stage.
 4. A method according to claim 1, whereinthe ammoniadesorbed in the second desorption stage is reabsorbed and pumped back tothe first desorption stage.
 5. A method according to claim 1, whereintheammonia desorbed in the second desorption stage is first brought incontact with volatilized ammonia formed after expansion of condensedammonia removed in the first desorption stage, and then the combinedammonia streams are compressed and returned to the first desorptionstage.
 6. A method according to claim 1, whereinthe absorption agentused in carrying out the absorption comprises ethylene glycol.
 7. Amethod according to claim 1, whereinsaid at least two stages of saiddesorption further include a third desorption stage in which additionalamounts of ammonia are desorbed from the desorption agent at a pressureof 3-15 bar.
 8. A method according to claim 1, whereincooling water usedfor desorption in the first desorption stage is subsequently used a thecooling water for condensing the ammonia desorbed in the firstdesorption stage.